1. Field of the Invention
This invention relates to optical non-uniformity correction (NUC) for active mode imaging sensors, and more particularly to optical NUC for angle of incidence between the scene and imager.
2. Description of the Related Art
Imaging sensors typically include a pixelated imager that is sensitive to incident photons within a certain spectral band e.g. Near IR (NIR), Short Wave IR (SWIR), Mid Wave IR (MWIR), Long Wave IR (LWIR), visible etc., imaging optics having an entrance pupil for receiving light from a scene within a field-of-view (FOV) of the sensor and imaging the FOV onto the imager, and a read out circuit for reading out an image of the FOV within the spectral band at a frame rate. The scene is composed of multiple point sources of light (collimated due to the distance to the scene) at different angles of incidence. The optics focus light from each point source onto the pixelated imager with the angle of incidence mapping to a spatial offset on the imager to form the image. Passive mode sensors detect emissions from the scene in the spectral band of the imager. Active mode sensors use a broad beam laser illuminator to illuminate the scene within the FOV of the sensor with laser energy in the spectral band to increase the signal-to-noise ratio (SNR). Generally, objects that emit in the spectral band will also reflect illumination of that band so there will be more signal from the object compared to the background.
Ideally an imaging sensor should respond uniformly across its FOV. That is the sensor response to a point source of illumination anywhere in the FOV should be the same. However, due to manufacturing issues of the pixelated imager, environmental variations and the radiometry of point sources, there are non-uniformities that exist in imaging sensors that must be corrected. Fixed pattern noise describes the non-uniformity in the response of the individual pixels of the imager itself. Notwithstanding the name, the fixed pattern noise can change with time and operating conditions such as temperature.
The irradiance at the entrance pupil of the imager from isotropic point sources across a plane varies with the 3rd power of the cosine of the angle of incidence from the optical axis of the imager to the point source. For a Lambertian point source (such as laser light reflected off objects in a scene), the irradiance varies with the 4th power of the cosine (See Chapter 2.3 Radiometric Approximations of The Art of Radiometry, SPIE Press Book, 2009). The angle may have a first component owing to the LOS from the imager to the scene and a second component owing to the spatial position of a point source within the FOV. The 3rd or 4th power of the cosine functions rolls off rapidly with the angle of incidence. Without correction, the sensor response will be highly non-uniform over the FOV, particularly if the LOS angle is appreciable.
Non-Uniformity Correction (NUC) may be performed as a calibration step at the time of manufacture and periodically in a laboratory setting. A black body source is used to produce flat field illumination across the sensor's FOV. The sensor's response is measured and used to calculate a scale factor for each pixel that is inversely proportional to that pixel's response so that the net response of the sensor is uniform over the FOV. This accounts for both fixed pattern noise and angle of incidence roll off over the FOV. Thereafter, the measured value for each pixel is multiplied by its scale factor, a form of “electronic” gain. An undesirable effect is scaling (increasing) the noise.
It is often desirable to periodically perform a NUC in an operational setting. To accomplish this the imaging sensor is provided with a black body source e.g. a small thermal source to provide flat field illumination. The sensor's response is measured and used to update the scale factors for each pixel. The operational NUC may account for changes in the fixed pattern noise or angle of incidence due to the LOS from the imager to the scene. In many applications, the LOS is constantly changing. The imaging sensor will lose visibility of the scene during NUC.
Another approach is to utilize a scene-based NUC in which the sequence of images (frames) are filtered and compared to determine and correct the non-uniformity. Scene-based NUC requires the post-processing of frames for filtering and comparison to previous frames. This process is computationally demanding and uses statistics to determine the non-uniformity of the scene, which could introduce errors in the final image. Scene-based NUC maintains visibility of the scene during NUC.